When a voltage is applied to an organic electroluminescence device (hereinafter, occasionally referred to as “organic EL device”), holes and electrons are injected into an emitting layer respectively from an anode and a cathode. The injected holes and electrons are recombined to generate excitons in the emitting layer. According to the electron spin statistics theory, singlet excitons and triplet excitons are generated at a ratio of 25%:75%.
A fluorescent organic EL device, which uses emission caused by singlet excitons, is inferred to exhibit an internal quantum efficiency of 25% at a maximum. Although having been used in full-color displays of a mobile phone, TV and the like, a fluorescent EL device is required to use triplet excitons in addition to singlet excitons to further enhance efficiency.
In view of the above, a highly efficient fluorescent organic EL device using delayed fluorescence has been studied.
For instance, a thermally activated delayed fluorescence (TADF) mechanism has been studied. The TADF mechanism uses such a phenomenon that inverse intersystem crossing from triplet excitons to singlet excitons thermally occurs when a material having a small energy difference (ΔST) between singlet energy level and triplet energy level is used. As for thermally activated delayed fluorescence, refer to, for instance, “ADACHI, Chihaya, ed. (Mar. 22, 2012), Yuki Hando-tai no Debaisu Bussei (Device Physics of Organic Semiconductors), Kodansha, pp. 261-262.”
For instance, Patent Literatures 1 to 3 disclose organic EL devices using the TADF mechanism.
Patent Literature 1 discloses an organic EL device including an emitting layer that contains a compound with a small ΔST as a host material and a fluorescent compound as a dopant material. According to Patent Literature 1, when the TADF mechanism is generated by using a compound with a small ΔST as a host material, the internal quantum efficiency is improved.
Patent Literatures 2 and 3 also each disclose an organic EL device including an emitting layer that contains a specific compound with a small ΔST as a host material and a fluorescent compound as a dopant material. In Patent Literatures 2 and 3, the TADF mechanism is used to improve the performance of the organic EL device as in Patent Literature 1.